In time divisional multiple access TDMA trunking system such as that being developed as TETRA (Trans European Trunked Radio) there are mobile stations which can operate in Trunked Mode (TMO) or Direct Mode (DMO). In TMO the mobile station communicates via a base-station whilst in DMO the mobile station establishes communication directly with another mobile station.
DMO is particularly useful where one mobile station is outside the area of coverage of the base-station, and is thus unable to use the trunking system, or if the trunking system has failed perhaps by the system crashing.
Usually TMO is used but the mobile station is provided with a switch to enable the operation to be switched to DMO manually or automatically.
A problem occurs where a talk group of a number of mobile stations is operating in TMO and a further mobile station moves outside the coverage of the trunking system still wishes to communicate with the group.
The talk group members will be unaware of the mobile station outside the coverage of the trunking system.
It is an object of the invention to provide a method by means of which a mobile outside the area of coverage of a trunking system can establish communication with a talk group operating in TMO.